Soil and Vegetation Fluoride Exposure Pathways to Cotton Rats on a Petrochemical-Contaminated Landfarm

Abstract

Abstract-Total fluoride (F) content of soils, vegetation, and cotton rats (Sigmodon hispidus) was measured on a landfarm contaminated with petrochemical wastes. A potentially bioavailable form of F was determined by HCl extraction of soils and vegetation. Cotton rats from the landfarm were examined for dental lesions indicative of fluorosis. Exposure pathways including dietary ingestion of unwashed vegetation and nondietary ingestion of soil associated with grooming and burrowing were evaluated. Mean bone F (1,515 mg/kg) and mean soil total F (1,954 mg/kg) from the landfarm site were greater than bone F (121 mg/kg) and soil total F (121 mg/kg) at a reference site. The HCl-extractable F was elevated in landfarm soil (326 mg/kg) compared to the reference site (2.3 mg/kg). About 80% of the cotton rats collected from the landfarm had dental fluorosis. During winter, the dietary pathway consisted of 78.9% of the potential bioavailable exposure (HCl-extractable F) of the two exposure pathways. However, in the summer, the nondietary pathway consisted of 87.9% of the potential bioavailable exposure of the two pathways. Incidental ingestion of soil associated with grooming and burrowing is more important than consumption of unwashed vegetation for cotton rats on the landfarm site in the summer. Fluoride accumulation in the soil from landfarming of petroleum wastes may pose a risk to terrestrial vertebrates. In addition to monitoring petroleum hydrocarbons, land application of petrochemical wastes should consider F and other inorganic contaminant loadings to the soil system.

Fluorosis Risks to Resident Hispid Cotton Rats on Land-Treatment Facilities for Petrochemical Wastes

“…We hypothesize that seasonally-induced changes in the feeding habits of cotton rats contribute to greater rates of soil ingestion and risks of fluorosis during winter. Modeling efforts by Schroder (1998) indicate that most uptake of fluoride compounds by cotton rats is from soil ingestion and dry matter deposition on plant material, and cool season grasses such as Bromus sp. contained higher concentrations of fluoride compared to warm season grasses.…”

“…Composite soil samples were mixed and sealed in acid-washed glass jars and transported to the laboratory for analysis. Total soil fluoride was analyzed for each composite soil sample by using fusion methods as described by McQuaker and Gurney (1977) and Schroder (1998). Briefly, 0.5 g of soil was placed in a 100 ml nickel crucible and slightly moistened with deionized distilled water.…”

“…A 1 ml aliquot was diluted to 5.0 ml with deionized distilled water and combined with 5.0 ml of TISAB II for fluoride determination with an combination ion selective electrode (Orion 960900, Orion Research Incorporated, Beverly, Massachusetts, USA) and reported as mg/kg on a dry weight basis. Aliquots of HNO 3 bone digest were analyzed for heavy metals and quantified by inductively coupled plasma emission spectroscopy (ICP; American Resources Laboratory, Fisons Maxim, Boston, Massachusetts, USA): Samples were analyzed for Ba, Cr, Pb, Sr, Ti, and Zn concentrations and reported as mg/kg on a dry weight basis (Schroder, 1998). Blanks containing just HNO 3 , spiked samples, and standard reference materials were used to evaluate laboratory procedures.…”

Soil Contamination and Bioaccumulation of Inorganics on Petrochemical Sites

“…The HCl-extractable F was 4-to 25-fold greater on the sites with elevated F compared to overall mean of reference sites; total F was 10-to 60-fold greater. In a detailed investigation of a landfarm, Schroder et al [10] found elevated levels of both total F (mean of 1,954 mg/kg) and HCl-extractable F (mean of 326 mg/kg) in soil that received application of oily sludges containing HF. All five landfarms (sites A through E) had elevated levels of both forms of F, suggesting that F in soil is more prevalent on landfarms than on other types of petrochemical sites.…”

“…Uptake of contaminants and transfer between trophic levels in small mammals for elements such as Cd, Pb, and F have been shown to occur on contaminated sites [3][4][5]. Small mammals have also been successfully used to document exposure and toxicity of both F and metals [6][7][8][9][10]. Cotton rats (Sigmodon hispidus) are indigenous to Oklahoma, USA, and serve a critical functional role in terrestrial food chains.…”